Passive RFID (radio frequency identification) tags are often used to identify and track objects in factories, warehouses, or supply chains. Passive RFID tags may comprise battery-free devices which are attached to objects. When queried by a wireless device called a reader, RFID tags may respond with their unique IDs, enabling the reader to read and identify them from a distance.
In a conventional passive RFID system, a reader bootstraps communication by sending a query on a downlink channel. The query triggers the tag to power-up and communicate messages to the tag. A powered-up tag responds with its unique identifier through ON-OFF keying modulation. To do so, the tag switches its internal impedance between two states: reflective and non-reflective.
In a conventional RFID system, localization techniques determine the location of a tag based on the phase of the received tag response, which encodes distance information.
In the downlink channel, after transmitting a query, a conventional reader transmits a continuous wave x(t) at some frequency f, which may be expressed as:x(t)=ej2πft  Eq. 1where t is time, and j is √{square root over (−1)}.
In the uplink channel, a conventional passive RFID tag modulates the wave x(t) with some signal s(t). The modulated signal y(t) that is reflected from the tag and received at the reader may be expressed asy(t)=s(t)ej2πf(t−2d/c)  Eq. 2where 2 d is the round-trip distance from the reader to the tag and c is the speed of light.
In a conventional RFID system, upon demodulation, a reader recovers the phase of the received signal as ϕ=4πd/c and use it for localization.
In a conventional RFID system, the communication range between the reader and the passive RFID tag is limited in the downlink channel. This is because, in a conventional system, the reader must be sufficiently close to power the passive RFID tag. In a conventional system: (a) the reader delivers power to the passive RFID tag (e.g., around −15 dBm for some passive UHF tags) so that the tag may power-up and decode. This limits the reliable range of conventional communication between a reader and passive tag to about 3-6 meters, because one cannot reliably power RFID tags at longer distances. The distance becomes much smaller if the RFID tag is buried under other objects, for example, under a stack of clothes in a retail store.
With conventional RFID systems, even if an entire store or warehouse were outfitted with a dense infrastructure of readers, 20-80% of the RFID tags may remain in blind spots due to destructive interference or orientation misalignment. Thus, in a conventional RFID system, to perform inventory control in a warehouse, an employee may walk around the warehouse carrying a reader (or maneuver a forklift, which carries a reader, around the warehouse) to scan for RFID tags throughout the entire warehouse, a process that may take up to a month.